Claims:

1. A phosphor (4) for lighting applications according to the formula
(Sr1-zMz)4Al14O25:Eu, Ln, Xk with M being
chosen from the group consisting of Ca, Ba, and Mg, Ln being chosen from
the group consisting of Dy and Nd, X being chosen from the group
consisting of Yb, Tm, and Sm, 0.ltoreq.z<1 and k ε {0; 1} and
k≠0 if z=0.

2. A method for the production of a phosphor (4) according to claim 1,
comprising the following steps: a) mixing raw materials which comprise
the elements of the phosphor (4); b) annealing the obtained mixture at
temperatures above about 900.degree. C. in a gaseous atmosphere.

3. The method according to claim 2, characterized in that the raw
materials comprise the metallic elements of the phosphor (4) as oxides
and/or carbonates.

4. The phosphor (4) according to claim 1, characterized in that the
phosphor (4) has been annealed in several steps, each step comprising the
application of a different gaseous atmosphere and/or a different
temperature.

5. The phosphor (4) according to claim 1, characterized in that the
phosphor (4) has been annealed in a gaseous atmosphere comprising air,
CO, N2 and/or H.sub.2.

6. The phosphor (4) according to claim 1, characterized in that the
phosphor (4) has been annealed at about 1300.degree. C. to 1500.degree.
C.

7. The phosphor (4) according to claim 6, characterized in that the
phosphor (4) has been annealed for between about 1 and about 6 hours.

8. The phosphor (4) according to claim 1, characterized in that
0.05.ltoreq.z≦0.15.

9. The phosphor (4) according to claim 1, characterized in that the
phosphor (4) comprises about 0.01 atom-% to about 10 atom-% Eu,
preferably about 1 atom-% Eu.

10. The phosphor (4) according to claim 1, characterized in that the
phosphor (4) comprises about 0.01 atom-% to about 10 atom-% Ln,
preferably about 0.05 atom-% Ln.

11. The phosphor (4) according to claim 1, characterized in that the
phosphor (4) comprises about 0.01 atom-% to about 10 atom-% X, preferably
about 1 atom-% X.

12. An illumination device (1) with a light source (2) and an afterglow
surface (4) that comprises a phosphor having an afterglow emission peak
at a temperature above about 100.degree. C.

13. An illumination device (1) with a light source (2) and an afterglow
surface (4) that comprises a phosphor according to claim 1.

14. The illumination device (1) according to claim 12, characterized in
that the afterglow surface (4) is arranged on a transparent cover (3) of
the light source (2), directly onto the light source (2), or on a carrier
(5, 6) of the light source.

15. The illumination device (1) according to claim 14, characterized in
that the phosphor is disposed as a layer (4) of a thickness between 1
μm and 1000 μm on the cover (3).

Description:

FIELD OF THE INVENTION

[0001] The invention relates to an illumination device with afterglow
characteristics. Moreover, it relates to a phosphor for lighting
applications and a method for its production.

BACKGROUND OF THE INVENTION

[0002] In US 2005/0242736 A1, an incandescent lamp is described with a
glass bulb that is coated with a phosphor to produce an afterglow effect
after the lamp has been switched off. The phosphor has the general
formula MAl14O25, where M is one or more of Ca, Sr and Ba.

SUMMARY OF THE INVENTION

[0003] Based on this background, it is an object of the present invention
to provide illumination devices with improved afterglow characteristics.

[0004] This object is achieved by a phosphor according to claim 1 and
illumination devices according to claims 8 and 9. Preferred embodiments
are disclosed in the dependent claims.

[0005] According to a first aspect, the invention relates to a phosphor
for lighting applications, particularly for illumination devices with
afterglow characteristics. The phosphor is composed according to the
following general formula:

(Sr1-z,Mz)4Al14O25:Eu, Ln, Xk (1)

[0006] wherein [0007] the variable M represents one of the
alkaline-earth metals Ca, Ba, and Mg; [0008] the variable Ln represents
one of the lanthanides Dy and Nd; [0009] the variable X represents one of
the lanthanides Yb, Tm, and Sm.

[0010] Furthermore, [0011] the index z is chosen from the interval [0, 1
[; [0012] the index k is either 1 or 0 (indicating that the component X
is present or not); [0013] k is not equal to 0 if z is 0, implying that
at least one of the components M and X must be present.

[0014] The above formula (1) describes a new phosphor which surprisingly
has advantageous afterglow characteristics. Experiments show that
afterglow is particularly improved for higher temperatures, for example
temperatures above 100° C. In practice this is very favorable as
such high temperatures often correspond to the operating temperatures of
illumination devices.

[0015] The invention further relates to a method for the production of a
phosphor of the kind described above, said method comprising the
following steps:

[0017] b) Annealing the obtained mixture at temperatures above about
900° C. in a gaseous atmosphere.

[0018] The raw materials that are used for the preparation of the phosphor
in step a) may preferably comprise the metallic elements of the phosphor
as oxides and/or carbonates. In particular, the raw materials may
comprise the compounds SrCO3, MCO3 (M=Ca, Ba, or Mg),
Eu2O3, Ln2O3 (Ln=Dy or Nd), X2O3 (X=Yb, Tm,
or Sm), and Al2O3.

[0019] Furthermore, the method may optionally comprise one or more of the
following steps: [0020] the addition of H3BO3 as a flux to
the mixture of step a); [0021] grinding the mixture of step a) with
acetone; [0022] milling the annealed mixture to obtain a fine powder of
the phosphor.

[0023] In the following, various embodiments of the invention will be
described that relate to both the phosphor and the method described
above.

[0024] Thus, the production of the phosphor of formula (1) preferably
comprises several annealing steps, wherein each step comprises the
application of a different gaseous atmosphere and/or a different
temperature. Most preferably, three such annealing steps are applied.

[0025] Moreover, the production of the phosphor of formula (1) may
optionally comprise annealing in a gaseous atmosphere comprising air, CO,
N2, and/or H2. Preferably, there are three annealing steps
taking place consecutively in the following different gaseous
atmospheres: air, CO, and N2/H2.

[0026] During its production, the phosphor according to formula (1) has
preferably been annealed at a temperature between about 1300° C.
and about 1500° C., preferably at a temperature of about
1400° C. Such annealing is typically executed as a final step of
the production process. Moreover, the duration of the annealing is
preferably in the range of about one to six hours.

[0027] According to a preferred embodiment of the invention, the index z
of the formula (1) ranges between about 0.05 and about 0.15. Most
preferably, z has a value of about 0.1±10%. It has been found that
such comparatively small fractions of the metal M can considerably
improve the afterglow characteristics of the phosphor.

[0028] Formula (1) for the phosphor does not specify the relative amounts
of the dopants Eu, Ln, and X. Preferably, these dopants are present
however in comparatively small fractions ranging between about 0.01
atom-% and 10 atom-%. Particularly preferred amounts are about 1 atom-%
for Eu, about 0.05 atom-% for Ln, and/or about 0.1 atom-% for X.

[0029] According to a second aspect, the invention relates to an
illumination device with a light source and an afterglow surface which is
illuminated by said light source and which comprises a phosphor having an
afterglow emission peak at a temperature above about 100° C.,
preferably above about 200° C. In this context, the "afterglow
emission peak" is determined by recording the emission intensity of the
phosphor as a function of temperature after exciting the phosphor at a
low temperature, wherein the temperature of the phosphor is raised at a
constant rate during the measurement. Typical rates at which the
temperature is raised during the measurement range between about 10 K/min
and 100 K/min and are preferably about 50 K/min. The described
measurement yields an "afterglow curve", wherein a peak of this curve (if
present) is by definition an "afterglow emission peak". Usually the
existence and location of an afterglow emission peak on the temperature
scale do not very critically depend on the particular rate of temperature
increase that is applied during the measurement.

[0030] The light source of the illumination device may be any component
that can actively generate light, for example a filament of an
incandescent lamp.

[0031] The described illumination device has improved characteristics
because the afterglow of its phosphor is high even at temperatures above
100° C. due to the existence of an emission peak in said range.
Afterglow is thus optimized at temperatures that correspond to the usual
operating temperatures of illumination devices, particularly of
incandescent lamps.

[0032] According to a third aspect, the invention relates to an
illumination device with a light source and an afterglow surface that
comprises a phosphor of the kind described above, i.e. a phosphor
according to formula (1).

[0033] An illumination device may preferably have the features of both
illumination devices according to the second and third aspect of the
invention, i.e. comprise a phosphor according to formula (1) that has an
afterglow emission peak at a temperature above about 100° C.

[0034] According to a further development of the above illumination
devices, the afterglow surface comprising the phosphor is arranged on a
transparent cover of the light source. Said transparent cover may for
instance be the glass bulb of an incandescent lamp. Arranging the
phosphor on a transparent cover has the advantage that light of the light
source may be transmitted through the phosphor (and the cover), thus
exposing the phosphor optimally to excitation illumination.

[0035] According to another embodiment, the phosphor is arranged on a
carrier (e.g. socket, basement) of the light source or even on the light
source (e.g. a filament) itself. These options have the advantage that
afterglow can originate from a location close to the light source, which
is however usually accompanied by the requirement to be resistant to high
operating temperatures.

[0036] In the aforementioned cases, the phosphor is preferably disposed as
a layer on the cover, said layer having a thickness between about 1 μm
and about 1000 μm, preferably between about 20 μm and 200 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] These and other aspects of the invention will be apparent from and
elucidated with reference to the embodiment(s) described hereinafter.
These embodiments will be described by way of example with the help of
the accompanying drawings in which:

[0042]FIG. 5 shows an incandescent lamp with a phosphor coating according
to the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0043] Afterglow pigments are mostly Eu2 doped aluminates or
silicates, which are co-doped with Dy3+ or Nd3+, resulting in
compositions such as SrAl2O4:Eu,Dy, CaAl2O4:Eu,Nd, or
Sr4Al14O25:Eu,Dy, wherein the observed afterglow is a
sensitive function of the type and concentration of the co-dopant.

[0044] FIG. 1 illustrates state transitions of electrons between the
valence band (VB) and the conduction band (CB) according to the most
widely accepted model to explain afterglow in Eu2+ doped aluminates.
This model involves oxygen vacancies as electron traps, which are located
close to Eu2+, which in turn act as deep hole traps (M. J. Knitel,
P. Dorenbos, C. W. E. van Eijk; J. Luminescence 72-74 (1997) 765). The
role of the trivalent co-dopant is the introduction of oxygen vacancies
and lattice distortions, which will give rise to the formation of oxygen
defects. Moreover, the most efficiently working trivalent ions as a
co-dopant to cause afterglow are Dy3+ and Nd3+, since these
ions easily act as hole traps, i.e. their redox potential for oxidation
to the tetravalent state is rather low.

[0045] Commercially available afterglow pigments, as given above, show
persistent afterglow at room temperature. However, an optimized afterglow
pigment for application onto light sources should show at least one glow
peak at a temperature above the temperature of the light source component
under operation on to which it is coated.

[0046] It is therefore proposed here to use phosphors exhibiting at least
one glow peak at a temperature above 100° C. (373 K), more
preferably above 200° C. (473 K), and to apply them onto (hot)
parts of light sources or luminaries.

[0047] Furthermore, it is proposed to optimize the persistent afterglow
pigment Sr4Al14O25:Eu,Dy by the replacement of Sr2+
with other alkaline-earth ions (Mg2+ or Ca2+ or Ba2+). It
was surprisingly found that the substitution of 10% Sr2+ with
Ca2+ gives a much more intense and persistent afterglow at room
temperature. FIG. 3 shows this in a diagram of the emission intensity
(vertical axis, in photon counts per second) of
(Sr1-zCaz)4Al14O25:Eu,Dy as a function of z and
time. It is assumed that this effect can be attributed to the formation
of a eutectic blend, resulting in a lower crystallization temperature of
the Sr4Al14O25 phase.

[0048] To improve the afterglow of (Sr,Ca)4Al14O25:Eu,Dy at
the temperature of a given application, e.g. at about 150° C., it
was found that its modification by the application of an additional
co-dopant is of advantage. An improvement of the persistence of the
afterglow at room temperature (FIG. 2) or at a high temperature, e.g. 150
or 300° C., is achieved by the addition of another trivalent rare
earth ion. It was surprisingly found that the application of Yb3+ as
an additional dopant improves the afterglow at room temperature, but it
also quenches the afterglow at a temperature above 150° C.

[0049] In contrast to the above, co-doping of
(Sr,Ca)4Al14O25:Eu,Dy with Tm3+ results in a slightly
worse afterglow at room temperature, but in a much more persistent
afterglow at a high temperature, e.g. at 300° C.

[0050] Finally, it was found that the persistence and intensity of the
afterglow of a given composition, e.g. of
(Sr,Ca)4Al14O25:Eu,Dy,Tm, is a sensitive function of the
synthesis temperature. The best results with respect to the afterglow
intensity and persistence are achieved if the final annealing step is
performed at about 1400° C.

[0051]FIG. 4 shows in a diagram the emission (expressed in counts per
second, vertical axis) along the so-called glow curves obtained by a TL
experiment. This means that the emission intensity is recorded as a
function of temperature T after charging the material at a low
temperature. During the experiment, the temperature T is linearly raised
at a constant rate, and the emission (TL) intensity is measured as a
function of temperature (i.e. as a function of time, since a temperature
ramp is applied).

[0052] The different curves represent the effect of the different
co-dopants (Tm, Sm, Yb) and of the temperature of the final annealing
step (1250° C., 1300° C., 1400° C.) according to the
following key:

[0058] In the following, various examples are provided to demonstrate
particularly selected embodiments of the present invention.

EXAMPLE 1

High Temperature Afterglow Pigment of the Composition
(Sr,Ca)4Al14O25:Eu(1%)Dy(0.05%)Tm(0.1%)

[0059] The required amounts of raw materials, i.e. 0.9265 g SrCO3,
0.0698 g CaCO3, 0.0124 g Eu2O3, 0.0007 g Dy2O3,
0.0014 g Tm2O3, 1.3307 g Al2O3, and 0.0109 g
H3BO3 as a flux were weighed in and ground with acetone in an
agate mortar. After drying of the blends they were filled into an alumina
crucible, which in turn was placed into a tube furnace. The material
underwent three annealing steps, which are

[0060] 1. step: Air/1000° C./4 h

[0061] 2. step: CO/1200° C./4 h

[0062] 3. step: N2/H2/1300° C./4 h

[0063] and was finally milled until a fine powder was obtained.

EXAMPLE 2

High Temperature Afterglow Pigment of the Composition
(Sr,Ca)4Al14O25:Eu(1%)Dy(0.05%)Sm(0.1%)

[0064] The required amounts of raw materials, i.e. 0.9265 g SrCO3,
0.0698 g CaCO3, 0.0124 g Eu2O3, 0.0007 g Dy2O3,
0.0012 g Sm2O3, 1.3307 g Al2O3, and 0.0109 g
H3BO3 as a flux were weighed in and ground with acetone in an
agate mortar. After drying of the blends they were filled into an alumina
crucible, which in turn was placed into a tube furnace. The material
underwent three annealing steps, which are

[0065] 1. step: air/1000° C./4 h

[0066] 2. step: CO/1200° C./4 h

[0067] 3. step: N2/H2/1300° C./4 h

[0068] and was finally milled until a fine powder was obtained.

EXAMPLE 3

High Temperature Afterglow Pigment of the Composition
(Sr,Ca)4Al14O25:Eu(1%)Dy(0.05%)Yb(0.1%)

[0069] The required amounts of raw materials, i.e. 0.9265 g SrCO3,
0.0698 g CaCO3, 0.0124 g Eu2O3, 0.0007 g Dy2O3,
0.0012 g Yb2O3, 1.3307 g Al2O3, and 0.0109 g
H3BO3 as a flux were weighed in and ground with acetone in an
agate mortar. After drying of the blends they were filled into an alumina
crucible, which in turn was placed into a tube furnace. The material
underwent three annealing steps, which are

[0070] 1. step: air/1000° C./4 h

[0071] 2. step: CO/1200° C./4 h

[0072] 3. step: N2/H2/1300° C./4 h

[0073] and was finally milled until a fine powder was obtained.

EXAMPLE 4

[0074] A solvent-based paint comprising
(Sr,Ca)4Al14O25:Eu,Dy,Tm as an afterglow pigment was
coated onto the basement of an automotive halogen lamp (H4 or H7). A
model of the lamp 1 is schematically shown in FIG. 5, and comprises the
filament 2, the glass bulb 3, the socket 5, and the coating 4 that covers
the inner surface of the bulb 3 and the basement 6 of the light source.
The thickness of the coating 4 was 20-200 μm. This lamp showed
blue-green (490 nm) persistent emission after the lamp had been switched
off.

[0075] Finally it is pointed out that in the present application the term
"comprising" does not exclude other elements or steps, that "a" or "an"
does not exclude a plurality, and that a single processor or other unit
may fulfill the functions of several means. The invention resides in each
and every novel characteristic feature and each and every combination of
characteristic features. Moreover, reference signs in the claims shall
not be construed as limiting their scope.